Activating mutations in the FLT3 receptor tyrosine kinase are the most common molecular abnormality in AML and are associated with significantly worse clinical outcomes. Several different small molecule FLT3 inhibitors, which vary considerably in selectivity for FLT3, have been studied in AML patients, and most have shown limited but consistent clinical activity. Our previous studies have demonstrated that FLT3 inhibition combined with chemotherapy leads to synergistic cytotoxic effects against FLT3 mutant AML cells, and that FLT3 mutations are present in leukemia stem cells (LSCs). Preliminary results from ongoing clinical studies of FLT3 inhibitors in relapsed AML patients suggest that chemotherapy followed by successful FLT3 inhibition leads to clinical benefit. However, it is not known how selective for FLT3 the inhibitors should be, when and for how long in the course of therapy they should be given, and whether or not they are effective against LSCs. We have previously succeeded in pre-clinically modeling AML treatment regimens incorporating FLT3 inhibitors, and this proposal aims to extend this work in the context of several different clinical trials. The broad goal of this proposal is to better understand how to incorporate FLT3 inhibition into AML therapy so as to improve survival or cure rates for FLT3 mutant AML. The specific goals will be to use primary leukemia cells from AML patients, including those enrolled on FLT3 inhibitor trials, to study the efficacy of selective and non-selective FLT3 inhibitors, alone and in combination with different sequences of chemotherapy, against bulk leukemia cells and leukemia stem and progenitor cells using in vitro and animal models. Plasma from patients enrolled on FLT3 inhibitor trials will be used to study the efficacy of FLT3 inhibition via measurement of FLT3 plasma inhibitory activity. The results of these studies using primary blast samples and plasma from trial patients will be correlated with clinical outcomes. Lay description: A gene known as FLT3 is mutated in the leukemia cells of about one third of acute myeloid leukemia (AML) cases, and this subset of patients has a very poor prognosis compared to those who lack this mutation. New drugs, known as FLT3 inhibitors, are being developed to target these FLT3 mutations. Our long-term goal is to learn how to incorporate these drugs into current treatment regimens in order to improve the cure rate and prolong survival for AML patients with FLT3 mutations. Many patients with acute myeloid leukemia (AML) have mutations in the FLT3 gene, which confer a worse prognosis. This proposal is aimed at incorporating a FLT3-targeted therapy into AML treatment so as to improve survival for these patients.